Spark-erosion machines, using a wire as electrode for cutting out an electrically-conducting part, are now in widespread use. The wires commonly used are formed from alloys of copper, zinc, steel, etc. and have diameters from 0.1 to 0.3 mm. The supply of new wire is stored on a reel that can contain several km of wire. The wire is unwound from the reel at speeds that can go from a few cm/s to 50 cm/s. It is guided into the machining area where it is subjected to a process that renders it unsuitable for further use: the wire is only used for machining once. In the case of some machines using spark erosion by wire, the same wire is re-used several times for machining; however, the present invention does not relate to these types of machines. In the framework of the present invention, the wire having passed through the machining region becomes a waste product and must therefore be conditioned in order to enable its recycling. One of the known means consists in cutting it up using any given cutting tool into segments of a few centimeters in order to limit its build-up and in order to be able to transport the residues into a receptacle of limited volume.
Numerous devices have been used in order to cut up the spent wire. The majority of them comprise mobile mechanical parts comprising cutting edges subject to wear. These parts, often made of special and costly materials, need to be replaced periodically. It should be added that the cutting devices produce repeated shocks which can make the wire vibrate in the machining region. They require relatively complex, costly and delicate mechanical assemblies to be developed and maintained. The present invention provides a more economical, simpler and more reliable solution.
The Patent text JP 61293727 describes a device for cutting up spent wire using an electrical current which causes the wire to melt. The wire is held under traction between two pulleys that pinch the wire. One of these two pulleys is made from an insulating material, the other comprises conducting sectors at its periphery. The current flows in the wire at regular intervals between the conducting sectors and a fixed contact disposed upstream. The latter does not provide a reliable contact with the wire. Another drawback is that the mechanical device is relatively complex; it comprises several moving parts subject to wear. Furthermore, the system makes continuous use of the fuse effect which causes the whole device to heat up.
In the Patent text JP 4193422 the principle is found of melting the wire by induction using a coil inside which the wire runs without making any contact. A high-frequency AC current, supplied by a power source, flows in the coil intermittently. The drawback of such a solution is that the high-frequency source is a relatively costly element since it has to also allow intense eddy currents to be induced in wires of very small diameter.
The patent text U.S. Pat. No. 5,523,538 mentions an element found in certain embodiments of the present invention. This is a long tube designed to carry the spent wire away from the machining region; an intense stream of compressed air is injected into this tube upstream in such a manner as to propel the wire. At the exit of the tube, the wire is recovered by a cutting device that chops it up into small segments. Here, there is an excessive complexity of the mechanisms and the drawback of the cutting tools which must be periodically replaced.
The Patent text EP 1634668 describes a wire cutting up system intended for a high-end machine. Special precautions are taken here to ensure that the cutting device does not interfere with the normal unwinding of the wire in the machining region. FIGS. 7a and 7b show a variant in which, at the exit of its disposal channel, the wire is deviated by a stream of compressed air, delivered by a nozzle 25, which propels it towards the blades 15,16. This example shows that it is necessary to provide alternative low-cost solutions for entry-level machines, which is achieved by the present invention.